Method and apparatus for thin metal film thickness measurement

ABSTRACT

A method for measuring a metal film thickness is provided. The method initiates with heating a region of interest of a metal film with a defined amount of heat energy. Then, a temperature of the metal film is measured. Next, a thickness of the metal film is calculated based upon the temperature and the defined amount of heat energy. A chemical mechanical planarization system capable of detecting a thin metal film through the detection of heat transfer dynamics is also provided.

BACKGROUND OF THE INVENTION

The invention relates generally to semiconductor fabrication and morespecifically to in-line metrology for process control during waferprocessing.

During semiconductor fabrication, in-line and in-situ metrology forprocess control and/or verification is commonly used. Inductive sensors,frequently referred to as eddy current sensors (ECS), were recentlyintroduced for semiconductor metrology for metal film thicknessmonitoring. Materials introduced from the alternative to the eddycurrent sensor side of the film helped to enhance dramatically sensorsensitivity especially in the thin film range, thereby allowing ECS toperform real-time measurements both in-situ by utilizing wafer carrierbuilt-in sensors, as well as stand-alone units for 3-CMP wafercharacterization.

However, one shortcoming with ECS at the enhanced sensitivity mode isthat the silicon substrate, as well as numerous other conductive objectslocated within the ECS sensing vicinity, contribute to the total signal.Consequently, the interpretation of the ECS readings introducesignificant uncertainty, especially in the low thickness range, whichplaces limitations upon the measurement capabilities especially forultra-thin reasonably high resistivity diffusion barrier films, i.e.,tantalum and tantalum nitride barrier films. Furthermore, the eddycurrents generated within the thin film and within the substrate areinductively coupled. This inductive coupling seriously affects the thinfilm lower sensitivity.

In view of the foregoing, there is a need to provide a method andapparatus that is capable of monitoring the barrier film thickness inorder to provide an in-line metrology device capable of providingaccurate thin metal film thickness.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing amethod and apparatus capable of providing a thickness of a thin metalfilm through heat transfer dynamics detected during the processing. Itshould be appreciated that the present invention can be implemented innumerous ways, including as an apparatus, a system, a device, or amethod. Several inventive embodiments of the present invention aredescribed below.

In accordance with one embodiment, a method for measuring a metal filmthickness is provided. The method initiates with heating a region ofinterest of a metal film with a defined amount of heat energy. Then, atemperature of the metal film is measured. Next, a thickness of themetal film is calculated based upon the temperature and the definedamount of heat energy.

In another embodiment, a method for determining a thickness of a metalfilm barrier is provided. The method initiates with delivering a definedamount of heat energy to a region of interest of the metal film barrier.Then, a heat transfer rate of the defined amount of heat energy alongthe metal film barrier is detected. Next, the thickness of the metalfilm barrier is determined based upon the heat transfer rate.

In accordance with yet another embodiment, a chemical mechanicalplanarization (CMP) system is provided. The CMP system includes a wafercarrier configured to support a wafer during a planarization process.The wafer carrier includes a sensor configured to detect heat energy. Animpulse heater configured to deliver a defined heat energy pulse to ametal layer disposed on the wafer is included. A computing device incommunication with the sensor is included. The computing device isconfigured to calculate a thickness of the metal layer based upon thedetected heat energy in relation to the defined heat energy pulse.

It is to be understood that the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof this specification, illustrate exemplary embodiments of the inventionand together with the description serve to explain the principles of theinvention.

FIG. 1 is a simplified schematic diagram of a chemical mechanicalplanarization (CMP) system configured to determine a metal filmthickness through heat transfer dynamics in accordance with oneembodiment of the invention.

FIGS. 2A through 2D are simplified schematic diagrams illustratingvarious configurations for the sensor and the heater in accordance withone embodiment of the invention.

FIG. 3 is a graph illustrating the relationship between a signaldetected by the sensor over time in accordance with one embodiment ofthe invention.

FIG. 4 is a flow chart of the method operations for measuring a filmthickness according to heat transfer dynamics in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several exemplary embodiments of the invention will now be described indetail with reference to the accompanying drawings. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will beunderstood, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

The embodiments described herein provide a scheme that is based onmonitoring metal thin film heat transfer properties in order todetermine a thin film thickness. It should be appreciated that the upperprocessed metal layer for a wafer will have high thermal conductivitywhich is normally deposited over a very poor thermal conductivedielectric layer, e.g., a silicon dioxide layer. Accordingly, the metalthin film is thermally insulated. As described below in more detail, aheat energy pulse delivered to a certain section of the metal layer willbe propagated mostly along the metal film. The heat energy pulse isessentially transparent to the substrate, over which the dielectriclayer is deposited, as well as being transparent to the dielectriclayer. The film temperature distribution is a function of film thicknessand thermal conductivity. Thus, by introducing a well-defined amount ofheat energy and then measuring the temperature of the thin film at thelocation where the heat energy was delivered, or at a certain distanceaway from this location, or even at the location after a certainpost-heat delay time, enables the capability of determining the filmthickness of the metal layer through heat transfer dynamics analysis.

FIG. 1 is a simplified schematic diagram of a chemical mechanicalplanarization (CMP) system configured to determine a metal filmthickness through heat transfer dynamics in accordance with oneembodiment of the invention. Wafer carrier 100, which rotates about itsaxis, includes heater 102 embedded therein. Wafer carrier 100 isconfigured to support wafer 118 during a chemical mechanicalplanarization (CMP) operation. Wafer 118 includes silicon substrate 112having insulated dielectric layer 116 disposed thereover, and metal filmlayer 114 disposed over the dielectric layer. Heater 102 is incommunication with controller 104, where the controller is also incommunication with heat sensor 106. During the CMP operation wafercarrier 100 supports wafer 118 against polishing pad 108 in order toplanarize metal film 114. Polishing pad 108 is disposed over stainlesssteel backing 110.

Still referring to FIG. 1, in one embodiment, heater 102 is configuredto deliver a pulse of infrared energy to a location defined on metalfilm layer 114. The infrared pulse may be converted into a heatradiation pulse, which may be infrared wide range, infrared singlewavelength, halogen bulb light, or any suitable heat radiation pulsethat induces a sharp, well focused and localized heating pulse. Sensor106 is configured to detect a signal indicating the temperature at thelocation where a heat pulse is delivered from heater 102. In anotherembodiment, sensor 106 is positioned to detect a signal indicating thetemperature of a location different from the location where the heatpulse is delivered. Thus, computation of the thickness of metal layer114 may be made based upon the amount of heat energy delivered to thelocation on the metal film and the temperature detected at the location,or nearby to the location, where the heat energy was delivered. Here,controller 104 is a computing device in which the parameters discussedabove are manipulated in order to determine the metal film thickness. Inone embodiment, controller 104 is a general computing device eitherstoring or having access to calibration curves generated through testwafers having different metal film thicknesses. The data of the blankettest wafers (BTW) or patterned test wafers (PTW) having different metalfilm thicknesses will correlate the temperature over time for a metalfilm layer having a certain thickness as discussed below with referenceto FIG. 3, based upon the amount of heat energy delivered from heater102.

FIGS. 2A through 2D are simplified schematic diagrams illustratingvarious configurations for the sensor and the heater in accordance withone embodiment of the invention. FIG. 2A illustrates a configurationwhere heater 102 and sensor 106 are located along axis 126 on opposingsides of wafer 118. Here, heater 102 delivers an energy pulse 120 ofheat energy to region 122 defined within metal layer 114 of substrate118. Region 122 may also be referred to as a region of interest. Sensor106 is configured to detect a signal indicating a temperature associatedwith region 122. As mentioned above, heater 102 may be an infraredimpulse heater. In this embodiment, the infrared impulse heater 102generates a pulse of infrared heat energy 120, which is transparent tosilicon substrate layer 112 and dielectric layer 116. As mentionedpreviously, dielectric layer may be any suitable thermally insulatingmaterial, e.g., silicon dioxide. Metal layer 114 is not transparent toinfrared energy pulse 120. As a result, metal layer 114 absorbs the heatenergy, which may be in the form of infrared energy. The absorption ofthe infrared energy causes a corresponding temperature increase atregion 122. This temperature increase is detected through sensor 106.

FIG. 2B represents an alternative configuration of FIG. 2A. Here, sensor106 and impulse heater 102 are offset from each other. That is, sensor106 and impulse heater 102 do not share the same axis as depicted inFIG. 2A. Thus, impulse heater 102 delivers a pulse of heat energy 120along axis 126 to region 122 within metal layer 114. Sensor 106 thendetects a temperature within metal layer 114 at region 124 which islocated along axis 128. It should be appreciated that in one embodiment,the sensing of temperature at region 124 is delayed from the delivery ofheat energy to region 122. One skilled in the art will appreciate thatthe delay may be induced through delay circuitry included with eithersensor 106 or the controller in communication with the sensor. It shouldbe appreciated that as the thickness of metal film 114 increases, thedissipation rate associated with heat energy delivered to region 122also increases. That is, since there is more metal to dissipate the heatas the thickness increases, the heat dissipation rate will be greater.Thus, for a given thickness, the heat dissipation rate, or thetemperature over time, may be captured in calibration curves defined byrunning BTW and PTW having metal film layer numerous thicknesses.Moreover, the temperature or heat dissipation rate may be recorded atvarious locations within metal film layer 114. This data is stored forsubsequent comparison with real-time data in order to determine acorresponding thickness associated with the metal layer based upon thedetected temperature and the level of heat energy pulse delivered to theregion of interest. It should be appreciated that the various locationsreferred to within metal film layer 114 may be offset from the region ofinterest. Thus, calibration curves corresponding to metal film layershaving various thicknesses are generated. Accordingly, the temperaturedetected by sensor 106 provides information in which the computingdevice mentioned above, with reference to FIG. 1, may use along with thegenerated calibration curves mentioned above, in order to determine thethickness of metal layer 114.

FIG. 2C illustrates another alternative configuration to FIG. 2A. Here,sensor 106 and impulse heater 102 are positioned along separate axes,128 and 126, respectively, as illustrated in FIG. 2B. However, sensor106 and impulse heater 102 are positioned on the same side of substrate118, rather than opposing sides. Controller 104, also referred to as ageneral computing device, is configured to trigger impulse heater 102 todeliver a predefined heat energy pulse 120 to region 122 of metal layer114. After a certain time period, controller 104 determines thetemperature within region 124 through a signal detected by sensor 106.As described above, controller 104 has access to data that correlatesknown thicknesses of layer 114 to heat dissipation rates ortemperatures, i.e., calibration curves. It will be apparent to oneskilled in the art that while sensor 106 and heater 102 are shown at thebottom side of substrate 118, the heater and the sensor may also beplaced at the top side of the substrate. FIG. 2D is yet anotheralternative embodiment to the configuration of the heater and sensor ofFIGS. 2A through 2C. Here, sensor 102 is located along axis 130 anddelivers heat energy pulse 120 to region 124 at an angle relative toaxis 128 associated with sensor 106. The angle between axis 130 and axis128 may be any suitable angle. In addition, sensor 106 does notnecessarily have to be normal to substrate 118. That is, sensor 106 maybe positioned at any suitable angle relative to the plane of the surfacesubstrate 118, In one embodiment, the angle of the axis of sensor 106relative to the axis of impulse heater 102 minimizes reflected heatenergy being detected by the sensor.

Still yet another embodiment may include at least two sensors configuredto detect a signal indicating a temperature, where the two sensorsdetect signals emanating from different locations. That is, twodifferent points away from the region of interest are monitored.Alternatively, two different delay times from the same spot may bemonitored, through a common sensor or through independent sensors. Ineither case, the heat dissipation constant may be determined withoutrequiring a reproducible heat inducing pulse from the impulse heater.One skilled in the art will appreciate that the heat dissipationconstant is a function of thickness and is calibrated in terms ofthickness. Therefore, through the heat dissipation constant, thethickness may be determined. The corresponding calibration curvesassociated with the two different points or the two different delaytimes will have the same shape but different amplitudes.

FIG. 3 is a graph illustrating the relationship between a signaldetected by the sensor over time in accordance with one embodiment ofthe invention. As can be seen, three curves are illustrated. Curve 130illustrates a response for a thin metal film, curve 132 illustrates aresponse for a thick metal film, and curve 134 illustrates the responsewhere the sensor is offset from the impulse heater. Thus, curves 130 and132 are associated with the configurations depicted in FIGS. 1, 2A and2D, while curve 134 is associated with the configurations depicted inFIGS. 2B and 2C. Curve 130 and curve 132 illustrate the relativetemperature rise for a thin metal film and a thicker metal film. Thatis, where the sensors of FIGS. 1 through 2D are configured to detect aninfrared signal, the signal associated with a thinner metal filmcorresponds to a greater initial temperature than the initialtemperature associated with the thick metal film. As mentioned above,the thinner metal film cannot dissipate the heat as quickly as a thickermetal film, due to the amount of metal capable for dissipating the heatenergy. Curve 134 illustrates the delay when the sensor is offset fromthe axis of the impulse heater and monitors a different region thanwhere the heat energy is delivered. Thus, with respect to FIGS. 2B and2C, the peak of curve 134 will move along the time axis as a function ofa distance between regions 122 and 124. Furthermore, with each of curves130, 132, and 134, the lower the temperature after a certain wait periodwill correspond to a thicker film. That is, the thicker film will have ahigher heat dissipation rate thus the temperature will be lower relativeto a thinner film where the same heat pulse is applied.

One skilled in the art will appreciate that numerous calibration curvesmay be generated through tests with BTW and PTW. The data generatedthrough these tests may be stored on a suitable storage medium for usein determining the thickness. Furthermore, the calibration curve datamay be incorporated into the sensor detecting the heat energy so thatthe signal may be translated at the sensor and converted to a thickness.

FIG. 4 is a flow chart of the method operations for measuring a filmthickness according to heat transfer dynamics in accordance with oneembodiment of the invention. The method initiates with operation 140where a region of interest of a metal film is heated with a definedamount of heat energy. Here, the region of interest corresponds to alocation on the metal film where a pulse of heat energy is delivered asdiscussed with reference to FIGS. 1 through 2D. In one embodiment, theheat energy is infrared energy. In another embodiment, the heat energyis substantially transparent to layers of the substrate except the metalfilm layer. The method then advances to operation 142 where thetemperature of the thin film is measured. In one embodiment, thetemperature of the thin film is detected through an infrared signal thatindicates a temperature associated with the region of interest. Asdescribed above, the sensor may be positioned to monitor the temperatureof the region of interest or a location separate from the region ofinterest.

The method of FIG. 4 then advances to operation 144 where a thickness ofthe metal film is calculated based upon the temperature and the definedamount of heat energy. In one embodiment, the calculation includesdefining a calibration curve correlating the temperature over timeassociated with a location on the metal film having a certain thickness.Thus, the thickness may be determined from the detected temperatureprovided by the sensor and associated time parameter. That is, thetemperature and time parameter will correspond to a point on a graphsuch as the graph of FIG. 3, thereby defining a thickness. Of course,the configuration of the sensor and the heater, i.e., whether they areoffset from each other will depend on which calibration curve is used todetermine the thickness.

In summary, the embodiments described above enable the use of heattransfer dynamics to extract information metal film thicknessparameters. Accordingly, barrier film thickness, e.g., tantalum andtantalum nitride barriers, may be accurately monitored through thedetection of the heat transfer dynamics described above. Thus, theobstacles of monitoring the barrier film thickness with eddy currentsensors have been overcome by exploiting the thermal properties of thethin films as described herein. As the film temperature distribution isdependent on time, coordinates, thermal conductivity, and thickness, thethickness may be determined when the other parameters are known. Thatis, the time of measuring the temperature relative to the delivery ofthe heat energy pulse is known, and the coordinates of the monitoringlocation relative to the location of delivery of the heat energy pulseare known. Thus, the thermal conductivity properties captured throughthe calibration curve generated from test wafers may be used to comparewith the thermal conductivity properties monitored from a wafer beingprocessed during a semiconductor processing operation where it isdesirable to determine the thickness of a thin metal film. It should beappreciated that the embodiments described above are capable ofdetecting a thin metal film having a thickness between about 5000Angstroms to about 10 angstroms.

One skilled in the art will appreciate that the embodiments describedherein may be applied as a process development tool. That is, duringqualification of a new tool, tests may be run to qualify the tool. Forexample, a CMP tool may be qualified to validate that the tool isperforming acceptably to monitor the thin metal film thickness asdescribed above. Additionally, while the embodiments are described withreference to the metal layer being thermally insulated, one skilled inthe art will appreciate that the embodiments may be extended to metalfilms disposed over any suitable non-conductive film or films. Theembodiments can also be extended to metal films disposed over anysuitable conductive film or films. Here, the heat transfer betweenlayers is taken into account.

In summary, the present invention provides for the generation andanalysis of a stress map associated with a substrate being processedduring a semiconductor processing operation. A proximity sensor, e.g.,an eddy current sensor, is used to detect a signal associated with alevel of mechanical stress being experienced at a location on thesubstrate. A temperature sensor, e.g., an infrared sensor, is used todetect a signal associated with thermal stress being experienced at thesubstrate surface. A stress map is then generated from multiple signals,in one embodiment. Analysis of the stress map reveals areas of thesubstrate experiencing stress conditions. Thereafter, corrective actionto relieve the stress condition is instituted. For example, if a hightemperature or high stress region is located on one portion of thesubstrate, processing parameters may be adjusted differentially torelieve the stress at the corresponding portion of the substrate.

It should be appreciated that while the embodiments have been describedin terms of a CMP process, the embodiments are not limited to a CMPprocess. For example, the sensors may be used within any semiconductorprocess that removes or deposits a layer or film on a substrate, such asetch, deposition and photoresist stripping processes. Furthermore, theabove described embodiments may be applied to rotary or orbital type CMPsystems as well as the belt type CMP system.

The embodiments described herein also provide for a CMP system that isconfigured to differentially control removal rates being applied toregions of a wafer. The differential control enables for a uniformthickness to be obtained as opposed to a uniform removal rate. Thedifferential control additionally allows for identified portions of thesubstrate having a high stress condition to be targeted for relief.

The invention has been described herein in terms of several exemplaryembodiments. Other embodiments of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention. The embodiments and preferred featuresdescribed above should be considered exemplary, with the invention beingdefined by the appended claims.

1. A chemical mechanical planarization (CMP) system, comprising: a wafercarrier configured to support a wafer during a planarization process; animpulse heater configured to deliver a single defined heat energy pulseto a metal layer disposed on the wafer; a sensor embedded in the wafercarrier, the sensor configured to detect heat energy emanating from alocation on the metal layer due to the heat energy pulse, the sensorlocated to minimize reception of a reflected heat energy pulse from thedefined heat energy pulse, the sensor is positioned to detect the heatenergy emanating from a location on the metal layer that is differentfrom where the impulse heater delivers the defined heat energy pulse tothe metal layer; and a computing device in communication with thesensor, the computing device configured to calculate a thickness of themetal layer based upon the detected heat energy in relation to thedefined heat energy pulse.
 2. The system of claim 1, wherein the sensoris an infrared sensor.
 3. The system of claim 1, wherein the heat energyis infrared heat energy.
 4. The system of claim 1, wherein the computingdevice includes a storage device storing a calibration curve relatingthe heat energy pulse and the detected heat energy to the thickness. 5.The system of claim 1, wherein the computing device is configured tocalculate a heat transfer rate from values associated with the detectedenergy and the defined heat energy pulse.
 6. The system of claim 1,wherein an axis of the impulse heater is different from an axis of thesensor.
 7. The system of claim 1, wherein the computing device includes,delay circuitry for delaying detection of the heat energy for a periodof time after delivering the defined heat energy pulse.
 8. A system fordetecting a thickness of a metal layer during a chemical mechanicalplanarization process without contacting the metal layer, comprising: aninfrared impulse heater configured to deliver a single defined heatenergy pulse to a metal layer disposed on a wafer; an infrared sensorconfigured to detect heat energy emanating from a location on the metallayer caused by the heat energy pulse, such that the sensor ispositioned to substantially eliminate reception of a reflected heatenergy pulse from the defined heat energy pulse, the location on themetal layer that the sensor detects the heat energy emanating from beingdifferent from a location where the impulse heater delivers the definedheat energy pulse to the metal layer; and a computing device incommunication with the sensor, the computing device configured tocalculate a thickness of the metal layer based upon the detected heatenergy in relation to the defined heat energy pulse.
 9. The system ofclaim 8, wherein an axis of the sensor and an axis of the impulse heaterare substantially orthogonal to a top surface of the metal layer. 10.The system of claim 8, wherein the computing device includes, delaycircuitry for delaying detection of the detected heat energy for aperiod of time after delivering the defined heat energy pulse.